As shown in FIG. 10, in a prior art gas dissolved water producing apparatus 201, a gas to be dissolved (hereafter referred to as solution gas) 103 is typically introduced into an outer side of a hollow fiber membrane 102 incorporated in a processing tank 111, and the gas is then dissolved through the hollow fiber membrane 102 into water to be treated 104 introduced into an inner side of the hollow fiber membrane 102, so as to produce a gas dissolved water 105.
The water to be treated 104 is fed by a pump 117 at a prescribed flow rate “q” to the inner side of the hollow fiber membrane 102 via a filter 118 and a flow meter 119. A flow rate signal 120 (illustrated by a broken line in the drawing) from the flow meter 119 is sent to a controller 121, which in turn sends a flow rate control signal 122 (illustrated by another broken line in the drawing) to a motor (not shown) for driving the pump 117, which is equipped with a revolution speed control unit (not shown). Therefore, the revolution speed of the motor can be controlled such that the pump 117 provides a prescribed flow rate “q”.
The solution gas 103 passes through a flow meter 116, where a flow rate is controlled to a prescribed level, and is sent to an outer side of the hollow fiber membrane 102 approximately under an ambient pressure. The gas is dissolved in water to be treated 104 through the hollow fiber membrane 102, while a residual gas is decomposed in an exhaust gas decomposing tower 130 with the aid of a catalyst (not shown) and is emitted as an exhaust gas 106.
In the conventional gas dissolved water producing apparatus discussed above, a concentration of a solution gas dissolved in a produced aqueous solution (gas dissolved water) is a saturated concentration of a solution gas under a pressure for supplying the solution gas (typically an ambient pressure, which is a cleaning pressure under which cleaning is performed by using the gas dissolved water). However, in precision machineries, electronic industries, and medical and food industries, gentle cleaning of workpieces may be required so as to avoid any damage. Thus, it is desired to produce a gas dissolved water having a concentration of solution gas in a gas dissolved water not greater than a saturated concentration at a cleaning pressure. The need for such a cleaning process is especially necessary for device wafers such as semiconductor wafers, which have micro-fabricated wiring. There is therefore an increasing demand for cleaning by use of functional water, such as water containing nitrogen, water containing ozone, or water containing oxygen, with the concentration controlled to a desired level below a saturated concentration.
Such a gas dissolved water can be produced by, for example, the following procedures. After the solution gas has been supplied to the outer side of the hollow fiber membrane, supply of the solution gas is suspended, with only the water to be treated being continuously supplied to dissolve the solution gas into the water to be treated. Consequently, pressure in the outer side of the hollow fiber membrane decreases, and the solution gas dissolves into the water to be treated and reaches a saturated concentration under the decreased pressure level. Once the concentration has reached a desired value, the solution gas is again supplied into the outer side of the hollow fiber membrane, such that the pressure level can be maintained. As a result, the concentration of the solution gas dissolved in the gas dissolved water is made lower than a saturated concentration in the case of supplying the solution gas under ambient pressure. However, disadvantageously, it takes a long time for pressure in the outer side of the hollow fiber membrane to reach the above-described pressure which is lower than ambient pressure. Besides, there is also a fear that a gas other than the targeted solution gas may become mixed with the solution gas under a pressure lower than ambient pressure.
The present invention has been made in light of the situation described above, and has as its object the provision of a gas dissolved water producing apparatus and a method therefor which is capable of producing a gas dissolved water having a concentration of a dissolved solution gas not greater than a saturated concentration under ambient pressure, both rapidly and efficiently.